Bone surgery

What is a partial knee replacement?

Partial knee replacement (also called unicompartmental knee arthroplasty - UKA) is surgery that may be used to treat severe knee arthritis that effects only one part of the knee. A knee surgeon may recommend knee joint replacement if someone has:

symptoms of knee arthritis, including knee pain and stiffness that keeps them up at night or prevents them from doing daily activities
knee pain that continues despite other treatments

Knee replacement surgery aims to reduce pain and restore function.

Total knee replacement is when the whole knee is replaced. This is for people with osteoarthritis affecting the whole knee. Some people with osteoarthritis that is limited to just one part (compartment) of the knee may be eligible for unicompartmental or ‘partial’ knee replacement.

In a partial knee replacement, only the damaged compartment is replaced with a new metal and plastic surface. The healthy cartilage and bone in the rest of the knee is left alone. Usually partial knee replacements use implants placed between the end of the thigh bone and the top of the shin bone. Some types of partial knee replacements can also replace the part of the joint under the kneecap.

A doctor explains what partial knee replacement involves and shows what the implants look like.

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Partial knee replacement involves taking away the damaged part of the knee and replacing that with some metal and plastic components. And the best way to demonstrate that would be to look at a model.

So this is a model of an Oxford partial knee replacement. And we can see that the metal and plastic components are replacing this inside part of the knee where the damage is. But unlike a total knee replacement, we’re going to preserve the ligaments and this normal part of the knee on the outside, which is functioning normally and doesn’t really need to be replaced. And so the essence of partial knee replacement is replacing the parts of the knee that are damaged and preserving the parts of the knee which are functioning well.

The key parts of the procedure are the implantation of this metal component on the femur and a metal component or plate, base plate, on the tibia. And in between there’s a small plastic component called the bearing. And this is, in essence, what a partial knee replacement is.

Partial knee replacement surgery involves a smaller incision (surgery cut) than traditional total knee replacement surgery. Many studies have shown that partial knee replacement works very well in most people who are suitable for it. The advantages of this type of surgery over total knee replacement include:

•   quicker recovery
•   less pain after surgery
•   less blood loss

Because the bone, cartilage, and ligaments in the healthy parts of the knee are kept, most people say that a partial knee replacement feels more ‘natural’ than a total knee replacement, and it usually bends much better.

A doctor talks about the advantages of partial knee replacement.

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The first thing is they’re a highly effective operation for getting rid of the pain and stiffness that you have in your knee. And for most people it’s a really big change in their quality of life. So what we call the treatment effect is a large treatment effect. And many patients are very happy with that.

The second advantage of partial knee replacement is, because a lot of the normal structures in your knee are preserved, that your recovery is faster. So you’re out of hospital sooner and you get going faster, and your recovery back to the level of function you want is faster.

It’s probably also true to say that, in terms of trying to get the best function you can after joint replacement, partial knee replacement can allow you to do that. Again because of preservation of the normal structures in the knee.

The next part of partial knee replacement which I think is attractive in terms of its advantages is a reduction in the type of complications that can occur following surgery. So we see a reduction in the amount of infection, blood clot, heart, lung and stroke problems following partial knee replacement compared to total knee replacement. And that’s an advantage for patients.

The potential disadvantages of partial knee replacement compared with total knee replacement include slightly less predictable pain relief, and the possible need for more surgery. For example, a total knee replacement may be necessary in the future if arthritis develops in the parts of the knee that have not been replaced.

A partial knee replacement operation usually lasts between 1 and 2 hours. As with any kind of surgery, there are risks involved. Although rare, the most common risks include blood clots, infection, injury to the nerves or vessels, continued pain, as well as the risks of an anaesthetic.

A doctor talks about the risks of partial knee replacement surgery.

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The risks of partial knee replacement are the risks that are associated with any surgery, but also the risks which are specific to this type of operation. Any surgery has a risk of infection, blood clot and medical complications such as heart problems, lung problems and stroke. But these problems occur in less than 1 per cent of all patients having this type of operation. So they’re rare occurrences.

There are some specific complications which are specific to partial knee replacement. And that is that the revision or failure rate can be a little bit higher than total knee replacement.

But on the whole most patients will find that their joint replacement lasts in the long term. And 90 per cent of patients are still functioning very well at 10 years following surgery. And 80 per cent are functioning well at 20 years following surgery.

The final, and perhaps the most important risk of partial knee replacement, and this is the same for any type of knee replacement, is that the operation doesn’t get rid of your pain in the way you would like it to. So around 5 per cent of patients will feel that they haven’t really achieved the level of recovery they would like to. They still have some residual pain and stiffness in the knee, and this may make them dissatisfied with the outcome of surgery.

Because a partial knee replacement is done through a smaller, less invasive cut, the time spent in hospital is shorter, and recovery and return to normal activities is faster than with a total knee replacement.

A doctor talks about recovering from partial knee replacement surgery.

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When we talk about the recovery from surgery, I think it’s a really good idea to break that up into sort of phases. So there’s the initial recovery in hospital, and patients now stay for 2, 3 days in hospital and in some cases will go home even the day after surgery with partial knee replacement.

And in that period of time your knee is still a little sore and swollen but you’re getting up, walking, perhaps with the aid of crutches. In many cases we’ll try and get you up on the day of surgery. And you’ll find that your mobility is not drastically affected at that point. You’re able to get around. But you are sore.

That gradually improves over the next 3 to 4 weeks. And we’ll see you again at the 6 week point in hospital, where we’ll check how you’re progressing. And we often find at that point that your mobility is much greater. You’ll often walk into the clinic very confidently.

And people may not even know you’ve had a joint replacement at that point. Your knee still is a little sore and swollen and can be puffy. But overall most people are already starting to feel the advantages of the procedure at that point.

If there are problems, then at the 6 week point we’ll review the situation and we’ll take an appropriate course of action to try and sort that out and improve the situation. If problems emerge earlier than that, or after the 6 week point of time, then through your General Practitioner you can always have access to the services in the hospital. And we’d be really keen to see you to try and sort that out.

In the longer term, it’s probably fair to say that the real benefit of joint replacement is enjoyed at approximately 6 months perhaps to a year following surgery. And that’s when a patient can expect their knee replacement to be functioning at its best. And that’s when they’ll enjoy the real return to function which we’re aiming for.

People who have a partial knee replacement usually have less pain after surgery, less swelling, and have easier rehabilitation than people having total knee replacement. In most cases, they go home 1 to 3 days after the operation. They can usually resume their regular daily activities 6 weeks after surgery.

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Last reviewed August 2018.